Lignocellulosic materials, such as wood, herbaceous material, agricultural residues, corn fiber, waste paper, pulp and paper mill residues, etc. can be used to produce ethanol. Generally, production of ethanol from lignocellulosic material requires four major steps. These four steps are pretreatment, hydrolysis, fermentation and recovery.
The first of these steps, pretreatment is also known as pre-hydrolysis. During this step the lignocellulosic material is heated to break down the lignin and carbohydrate structure, solubilize most of the hemicellulose and make the cellulose fraction accessible to cellulase enzymes. This heating is done either directly with steam or in slurry. Also, a catalyst may be added to the material to speed up the reactions. Catalysts suitable for this include strong acids, such as sulfuric acid and SO.sub.2, or alkalis, such as sodium hydroxide.
The second step is hydrolysis, more specifically enzymatic hydrolysis. After the pretreatment step, enzymes are added to the pretreated material to convert the cellulose fraction to glucose. This is also known as saccharification and is generally done in stirred-tank reactors or fermentors under controlled pH, temperature and mixing conditions.
The third step is fermentation of the sugars to ethanol. The sugars, released from the material as a result of the pretreatment and enzymatic hydrolysis, are fermented to ethanol by a fermenting organism, such as yeast, for example. The fermentation can also be carried out simultaneously with the enzymatic hydrolysis in the same vessels, again under controlled pH, temperature and mixing conditions. When saccharification and fermentation are performed simultaneously in the same vessel, the process is generally termed simultaneous saccharification and fermentation or SSF.
The fourth step is the recovery of the ethanol from the fermentation broth by distillation.
The enzymatic hydrolysis and fermentation processing steps have the following common requirements, particularly when the cellulosic material is in the form of a slurry:
The slurry is maintained at a set temperature for a predetermined time.
Adequate mixing is required to ensure effective and uniform heat and mass transfers. However, overly vigorous mixing can damage and denature the enzymes and fermenting organisms due to high shear. See Shear Inactivation of Cellulase of Trichoderma ressei by Reese and Ryu, Enzyme Microb. Technol., July, 1980, Vol. 2, p. 239-240 and Effects of Agitation on Enzymatic hydrolysis of Cellulose in a Stirred-Tank Reactor by Mukataka, Tada and Takahashi, Ferment, Technol., 1983, Vol. 61, no. 6, p. 615-621. Also, vigorous mixing requires large agitators and considerable power consumption which, in turn, significantly affects the economics of plant operation. Such economic considerations are quite considerable. For example, for a 2,000 ton/day plant, the capital cost of the SSF operation using continuous stirred-tank reactors (CSTR) in series was estimated at 16% of the fixed capital investment. See Preliminary Estimate of the Cost of Ethanol Production for SSF Technology by Hinman et al., Appl. Bio. and Biotech., 1992, Vol. 34/35, p. 639-649. This value represents the third largest investment after pretreatment and utilities capital costs. The high capital cost of the fermentors is attributed to the large total SSF volume resulting from a typical 4-5 day retention time needed to complete the hydrolysis and fermentation using continuous stirred tank reactors connected in series. Based on pilot plant data and using a scale up exponent of 0.3, the mixing power requirement to keep pretreated sawdust particles (10 wt % insoluble solids) in suspension in a 1 million liter fermentor is estimated at 120 HP (or 0.5 HP/1000 gal). The estimated increase in mixing cost is $0.05/gal of ethanol when compared to the target mixing power of 0.1 HP/1000 gal. See Technical and Economic Analysis of an Enzymatic Hydrolysis Based Ethanol Plant--Draft by. Schell et al., 1991, SERI TP-232-4295, p. 54-55. This cost increase is significant for a process that is targeting $0.67/gal of ethanol as a cost goal. The projected high mixing energy requirement not only is costly but also presents a challenge in scaling up to very large-sized fermentors of 1 million gallons or larger, since the heat generated by the SSF process and by the agitators would be difficult to remove without using extensive cooling loops designed specifically for slurry. Depending on the extent of the cellulose hydrolysis and the lignin content in the material, the insoluble solid concentration in the SSF fermentors would gradually drop from about 12 wt % to only about 4 wt % in the last fermentor. As a result, there is a significant cost savings if the total volume of the fermentors and the mixing power requirement are reduced. These two factors form the basis for this invention.